The present invention relates to an image input apparatus utilizing a one-dimensional image sensor.
Conventionally, there is known an image input apparatus utilizing a one-dimensional CCD (charge-coupled device) image sensor, in which pixel signal shift clocks more than the number of pixels which this image sensor holds are supplied per one scanning, and then all the pixel signals of the image sensor are shifted to thereby read out an image.
FIG. 6 is a block diagram showing a conventional image input apparatus, in which reference numeral 1 denotes a manuscript; 2, a manuscript conveying mechanism constituting manuscript conveying means by conveying the manuscript 1 in a conveying direction 3 at a preset speed; 4, a conveying speed control section that adjusts the speed of conveying the manuscript conveying-mechanism 2 to a desired speed; 5, a light source for irradiating light with a desired illumination to the manuscript 1; 6, a lens for imaging image information obtained from the manuscript 1 onto a one-dimensional CCD image sensor 7; 7a, 7b and 7c, one-dimensional CCD image sensors each having different number of read-out pixels (number of photocells); 8, a driving signal control section that outputs pixel signal shift clocks that successively shift the output signals from photocells, and outputs main scanning signals; 9, a gain control section that adjusts the gain of output signals from the one-dimensional CCD image sensors 7a, 7b and 7c.
Now, the description is made of an operation of a conventional example having the above construction.
First, prior to a read-out of a manuscript, an image sensor having the number of photocells corresponding to the size of a manuscript in a main scanning direction is selected among one-dimensional CCD image sensors 7a, 7b and 7c, where, for example, the one-dimensional CCD image sensor 7b is selected.
Next, the manuscript 1 is placed on the manuscript conveying mechanism 2 with its read-out surface turned up, and the manuscript 1 is conveyed at a preset conveying speed along the conveying direction 3 with irradiating light from the light source 5 having a desired illumination onto the read-out surface of the manuscript. This conveying speed is set in the conveying speed control section 4 in advance. Then, the main scanning signal is supplied to the one-dimensional CCD image sensor 7b from the driving signal control section 8 to read out the image information in the manuscript 1 through the lens 6. The read-out image is supplied to the gain control section 9 by shifting one pixel after another by the pixel shift clock supplied from the driving signal control section 8. The image information is amplified to the desired level at the gain section 9 and output as image signals.
FIG. 7 is a time chart showing the main scanning signal and the pixel shift clock in the conventional image input apparatus. TINT0 is a time required for one scanning, .DELTA.t is a minimum time required for shifting one pixel signal, and N.sub.o is the number of all the photocells constituting the one-dimensional CCD image sensor 7b.
Here, the time required for one scanning TINT0 has the relation with the number of photocells in the one-dimensional CCD image sensor 7b as shown in the following equation: EQU TINT0.gtoreq..DELTA.t.times.N.sub.o
That is, the time required for one scanning TINT0 has a lower limit (minimum value) of .DELTA.t.times.N.sub.o.
Thus, the conventional image input apparatus has such a problem that since the time required for one scanning TINT0 has a lower limit, there is an upper limit in a speed of conveying a manuscript, and also a limitation in the speedup of image inputting.
Also, when image information is read out so a line density in the main scanning direction as to be consistent irrespective of a size of manuscript, there is a problem that a plurality of image sensors with different number of photocells are required to be prepared depending upon the sizes of manuscripts.